1. Field of the Invention
The present invention relates to an image forming apparatus capable of forming a latent image on a latent image bearing means by means of an exposure means and a developing apparatus employed therein, and more particularly to a copying apparatus, a printing apparatus, a facsimile apparatus or the like of the BAE system to be explained later, equipped with an amorphous silicon photosensitive member as the electrostatic latent image bearing member.
2. Related Background Art
The image forming apparatus of a digital system forms the image by a process to be explained in the following. FIG. 1 is a schematic cross-sectional view of such image forming apparatus.
A drum-shaped photosensitive member 1, constituting an electrostatic latent image bearing means, is uniformly charged with a primary charging means 3 at a potential Vdpre (V). A laser beam B, induced by a laser driver circuit 81, is directed on an unrepresented rotary polygon mirror, called a polygon scanner. The beam B reflected from the rotary polygon mirror irradiates the surface of the electrostatic latent image bearing means 1, displaced at a constant velocity Vprs. By the rotation of the rotary polygon mirror, the laser beam B scans the photosensitive drum 1 in a direction A of movement of the photosensitive drum 1 (hereinafter called subscanning direction) and a perpendicular direction K (hereinafter called main-scanning direction) (cf. FIG. 12). The laser beam B is turned on and off according to the image to be outputted, thereby forming an electrostatic latent image on the photosensitive drum 1, and such electrostatic latent image is developed into a visible toner image.
The toner image is transferred by transfer means 5 onto a transfer material M, which is then separated by separation means 6 from the electrostatic latent image bearing means 1 and the toner image is fixed onto the transfer material M by unrepresented fixing means to obtain the final print output. The toner T' not transferred onto the transfer material M but remaining on the electrostatic latent image bearing means is removed by a cleaning means 7.
In the first place, in order to obtain a clear image without fog or the like, the toner is required to have an appropriate electrostatic charge. The toner is also required to be free from changes such as significant attenuation of the charge amount or solidification caused by a secular change or an environmental change such as change in humidity. This is because an attenuation of the charge amount from the initially set value results in various drawbacks such as a decrease in the image density, an increase in toner scattering leading to background fog or toner scattering on the white image area, and smearing with toner around the developing means 4.
In order to meet the above-mentioned requirements, the addition of a charge controlling agent is almost essential in manufacturing toner, but, with the recent increase of color image formation, there is required a charge control agent of a white or pale yellow color in order to achieve satisfactory color reproducing ability.
Among the negative charge control agents for providing a negative charge, the colorless, white-color and pale yellow-colored ones are already commercially available with satisfactory performance and can therefore be adopted without any practical limitation. On the other hand, for the positive charge control agents for providing a positive charge, particularly those suitable for use in colored toner, there are only known nigrosin dyes as colored ones and quaternary ammonium salts or imidazole compounds (Japanese Patent Application Laid-open Nos. 62-287262, 61-269265 and 59-187350) as colorless ones and they are quite limited in their kinds.
The nigrosin dye is not composed of a pure compound but of a mixture of several compounds with an unidentified composition, so that a constant performance cannot be expected. Also the nigrosin dye is effective for giving a stable charge to the toner for use in a copying apparatus of a low or medium transfer speed, but cannot exhibit such function in the toner composed of a low melting point and a low viscosity for a high-speed copying apparatus. Also the nigrosin dyes, being colored, are not suitable for use in the colored toners.
The quaternary ammonium salts have various drawbacks such as an environmental instability of the charging ability, a large particle size prohibiting the fusion at the fixing operation and an unpleasant small. Also the quaternary ammonium salts are unsatisfactory in the charging ability, like the nigrosin dyes.
Thus the positive charge control agents for providing a positive charge are limited in the kinds, and are particularly limited in colorless or white-colored ones usable for the color toners. Consequently the negatively charged toner has to be considered to have a larger freedom at present, and in practice such negative charged toner is widely employed in the high-speed image forming apparatus.
On the other hand, in the photosensitive drums, organic photoconductors (hereinafter represented as OPC) are widely employed because of a lower manufacturing cost and absence of toxicity.
However the OPC shows fast abrasion and a nonlinear relationship between the exposure energy Eexp and the photosensitive drum potential Vs as shown in FIG. 13, whereby the potential sags at the edge portion of the image, resulting in thinning of the image lines in the BAE system.
Also the OPC is associated with the significant deterioration of the charging characteristics and the exposure characteristics with the repetition of the image outputs (FIG. 14), wherein a indicates the deterioration of the charging characteristics (potential decrease in the dark image area) and b indicates the deterioration of the photosensitive characteristics (potential increase in the light image area).
It will be understood that the development contrast Vcdev, which is initially 500 (V), is reduced to 300 (V) at the end. It is already known that such a decrease in the development contrast Vcdev results in a decrease in the developed toner amount, and also in thinning of the image lines.
On the other hand, the electrostatic latent image bearing means consisting of amorphous silicon (hereinafter represented as a-Si), though higher in the manufacturing cost, can form the latent image relatively faithful to the distribution of the exposure energy, so that the image quality is not much different between the IAE and BAE systems. Also because of the drastically higher durability, the cost per print eventually becomes lower, so that such latent image bearing means is ideal for the high-speed image forming apparatus.
In forming digital electrostatic latent image, there are known the following two systems.
The first system, IAE (image area exposure) system for effecting laser exposure in the dark image area, is widely employed in the digital copying apparatus, printers etc. utilizing the laser beam for latent image formation.
In the IAE system, the potential V1 of the light image area (white area) and the potential Vd of the dark image area (black area) satisfy a relationship .vertline.Vd.vertline.&gt;.vertline.V1.vertline., and the toner image is formed by reversal development in the developing means with toner charged with a polarity same as that of the electrostatic latent image bearing means, under a developing bias of an average value Vm.
The second system, BAE (background area exposure) for effecting laser exposure in the light image area (white area), is widely employed in the analog image forming apparatus such a copying apparatus.
In the BAE system, then potential V1 of the light image area (white area) and the potential Vd of the dark image area (black area) satisfy a relationship .vertline.Vd.vertline.&gt;.vertline.V1.vertline., and the toner image is formed by normal development in the developing means with toner charged in a polarity opposite to that of the electrostatic latent image bearing means, under a developing bias of an average value Vm.
The BAE system can be realized also in the digital manner, besides in the analog manner, by reducing the absolute value of the potential in the white area other than the image information area, for example with a laser beam.
Consequently, in the high-speed image forming apparatus, the combination of the negatively charged toner and the a-Si based electrostatic latent image bearing means is ideal.
Such image forming apparatus inevitably utilizes the latent image formation by the BAE system. However, the intensity distribution of the laser beam scanning the surface of the electrostatic latent image bearing means 1 is not completely rectangular, as indicated by a broken line in FIG. 8, but shows a Gaussian distribution as indicated by a solid line, so that, in the laser exposure in the BAE system, the light spot at the boundary of a dark area and a light area spreads into the dark image area (black area). As a result, in a fine line image or in fine image spots, the electrostatic latent image becomes finer than the actual image information, so that the toner image after image development cannot is faithful to the image information.
Also if the background contrast potential Vcbg, which is the difference between the average developing bias Vm and the light potential V1 of the electrostatic latent image bearing means, is small, the insufficiently charged toner tends to be deposited in the area where the charge is not completely dissipated between the sub scanning lines (FIG. 15). Consequently, in order to reproduce a fine line, the spot diameter has to be sufficiently small, and the contrast between the dark potential and the light potential inevitably becomes smaller even if the striped fog in the sub scanning direction becomes less conspicuous, so that the density of the solid black area is lowered. Such image development becomes more conspicuous as the resolution of the latent image is elevated.